Systems for providing image signals typically include a camera positioned to be focused upon an object and constructed for providing image signals representing an image of the object. Typically, these systems include cameras having an array of discrete charge coupled devices ("CCDs"), referred to as pixels, for providing electrical signals wherein the magnitude of the electrical signal is indicative of the intensity of light incident upon each pixel. By properly timing the manner that the plurality of signals are obtained from the camera, the plurality of signals can be used to represent an image of an object upon which the camera is focused.
However, several factors contribute to fixed pattern variations of the plurality of discrete pixel elements imaged by the camera. One factor, referred to herein as illumination variation, results from nonuniformity in illumination caused by optical element imperfections, dust collection, and positioning variations. Another factor, referred to herein as CCD variation, results from two types of imperfections in the performance of the CCDs. One performance imperfection results from size tolerance variations on the active area sensitive to light for each pixel. These size imperfections result in sensitivity variations between pixel on a given sensor that are mathematically equivalent to illumination variation. Illumination variation, whether caused by nonuniformity in illumination or by CCD size variations are corrected for using gain correction.
Another factor that contributes to fixed pattern variations is also caused by CCD performance imperfections. This factor is referred to as offset variation and is caused by the dark current signal within the CCD, i.e., the current signal that is present at the CCD when no light is incident on the CCD. The offset variation is determined as the magnitude of the signal from a pixel when no light is incident upon the pixel. Illumination variation and offset variation present themselves as two types of errors in the resulting signal from each pixel.
It is desirable, therefore, to provide method and apparatus for correcting image signals from a camera for illumination variation, resulting from nonuniformity in illumination and from CCD size variation. It is also desirable to provide method and apparatus for correcting image signals from a camera for offset variation resulting from CCD dark current.
Further, in order to obtain a plurality of signals from the array of pixel elements that most accurately represent an image of the object, it is necessary to correct each pixel for the above mentioned variations in offset and illumination. Most prior art systems for collecting image signals have attempted to provide data processing methods for correcting the plurality of signals. However, these methods cannot be performed at the same speed that the signals are obtained from the camera and, therefore, introduce significant delay in the time required to obtain an image signal. Further, these methods are also not accurate. Accordingly, it is desirable to provide method and apparatus for correcting a plurality of signals from a CCD camera wherein the correction can be made rapidly while the plurality of signals are obtained from the camera and wherein the correction is made prior to the analog-to-digital conversion, to utilize the maximum number of quantization steps to measure signal levels.